The National Institute of Allergy and Infectious Diseases (NIAID) has a strong commitment to the maintenance of animal models for the studies of both basic science and human disease. For the last thirty years, NIAID has supported programs leading to the creation of many inbred mouse strains, including congenics, recombinant inbred, mutants and more recently, transgenics. These mouse strains have been extremely valuable in the delineation of important genetic and immunologic principles, in particular the role of the major histocompatibility complex in the genetic control of the immune response. Although many of the conventional and recombinant mouse strains which previously had been in high demand are now used less frequently, they still remain a valuable genetic resource. The cost of maintaining breeding colonies has increased so dramatically in recent years that the widespread availability of many unique lines is now threatened. This has already resulted in and will continue to result in the loss of some valuable congenic and recombinant strains. The increasing demand on mouse breeding facilities has been exacerbated by the recent development of transgenic mice. These new lines represent a unique biologic resource; however, they are both difficult and expensive to construct. Cryopreservation of mammalian embryos has proven to be a dependable method for the long term preservation of important genetic stocks since the methodology was first developed in 1972. Unique murine strains, which are now used infrequently, may be maintained efficiently, and at less cost, as frozen embryos, rather than in small breeding colonies. This contract would fund the maintenance of a mouse embryo storage facility, similar to the NIAID hybridoma bank and HLA sera repository, which would obtain appropriate strains for cryopreservation, breed, prepare, store, and reconstitute frozen embryos.